This invention relates to magnetic devices, and more particularly to magnetic devices including exchange biased thin films.
The pursuit of high areal density magnetic recording toward 100 Gbits/in2 has prompted extensive research in the area of new magnetic materials with perpendicular anisotropy. For thin film fabrication, this becomes a formidable challenge since the magnetostatic energy favors keeping the magnetization in the plane of the film. While attention has focused primarily on recording media, either alloys or multilayer devices, little or no emphasis has been placed on spin valve or tunnel junction devices possessing a perpendicular anisotropy.
Spin valves are widely used as magnetic sensors. Spin valves utilize an antiferromagnetic (AF) layer to pin the direction of magnetization in a ferromagnetic (FM) layer by a phenomenon known as exchange bias. Biasing arises from an interfacial exchange coupling between a FM layer in direct contact with an AF layer. The exchange bias effect is easily observed from the hysteresis loop being offset from zero field by an amount known as the exchange field (HE) together with an increase in the coercivity (HC). Since its discovery over 40 years ago, exchange bias has since been observed in a number of magnetic bilayer systems. A key to understanding the nature of this effect is the interfacial spin device in both the FM and AF layers. However, despite all the experimental and theoretical work to date, the exchange bias effect remains unresolved.
There is one common approach to all exchange coupled AF/FM bilayer studies; that is, the magnetization of the FM layer is confined to lie in the plane of the film. Recently it was shown that perpendicular exchange bias can be observed in a (Co/Pt) multilayer grown on an AF FeF2 substrate. At low temperatures, a shifted hysteresis loop has been observed similar to the in-plane exchange bias systems with the distinction that the magnetic field is applied normal to the substrate plane. Subsequently other studies were also reported using the same type of (Co/Pt) multilayer with an oxidized Co capping layer, where CoO is the AF layer. These examples introduced a novel experimental approach to the exchange bias problem and shed new light on the nature of the spin device between the FM and AF layers. However, in those two studies the FM component was made up of a rather complex multilayer device where aside from the exchange bias effect, a different type of interfacial exchange coupling between the Co and Pt layers is also present.
There are numerous thin film ferromagnets that possess a perpendicular anisotropy. In these cases, the FM layer exhibits a spin reorientation effect due to distortion of the crystal lattice by epitaxial growth on an appropriate underlying substrate. Some common examples are Co/Au (111) and Ni/Cu (002). Ni on Cu is one of the most widely studied systems having many salient features that are ideal for an exchange bias study. Fundamentally, Ni films greater than 20 xc3x85 thick are magnetic at room temperature and the perpendicular anisotropy has been shown to extend over a 100 xc3x85 range. HC values less than 50 Oe have also been reported for films less than 40 xc3x85 thick.
A device with the magnetization normal to the substrate plane may offer new and attractive features for magnetic field detection. Thus this invention provides a perpendicular exchange bias device that can be used for magnetic field detection.
Perpendicular exchange biased device constructed in accordance with the invention comprise a layer of buffer material on a surface of a substrate, a layer of ferromagnetic material on a surface of the buffer layer, wherein the magnetization of the ferromagnetic layer lies in a direction perpendicular to the plane of the layer of ferromagnetic material, and a layer of antiferromagnetic material on a surface of the layer of ferromagnetic material.
The buffer material can comprise a material selected from the group of copper and diamond. The layer of ferromagnetic material can comprise a material selected from the group of nickel or an alloy containing nickel. The layer of antiferromagnetic material can comprise a manganese-based alloy, for example FeMn.
The buffer material can alternatively comprise (002) copper or (001) diamond. The diamond can be boron-doped diamond or nitrogen doped diamond.
The invention also encompasses a method of making a perpendicular exchange biased device comprising positioning a layer of buffer material on a surface of a substrate, positioning a layer of ferromagnetic material on a surface of the layer of buffer material, wherein the magnetization of the ferromagnetic layer lies in a direction perpendicular to the plane of the layer of ferromagnetic material, and positioning a layer of antiferromagnetic material on a surface of the layer of ferromagnetic material.
The buffer material comprises a material selected from the group of copper and diamond. The layer of ferromagnetic material can comprise a material selected from the group of nickel or an alloy containing nickel. The layer of antiferromagnetic material can comprise a manganese-based alloy, for example FeMn.
The buffer material can comprise (002) copper or (001) diamond. The diamond can be a boron-doped diamond or a nitrogen doped diamond.
The step of positioning a layer of ferromagnetic material on a surface of the layer of buffer material can comprise the step of epitaxially growing the layer of ferromagnetic material on the surface of the layer of buffer material.